Computerized systems, from simple appliances to complex networks, contain many different parts, such as displays, keyboards, mice, printed circuit boards, drives, switches, modems, processors, and memory, etc. Memory is capable of retaining digital information under certain conditions. The retained information might be operational code or data files, or a combination of the two. An ideal memory subsystem optimizes density, preserves critical information in a nonvolatile condition, is easy to program and reprogram, can be read fast, and is cost-effective for a specific application.
Electrically erasable and programmable read-only memory (EEPROM) is a nonvolatile memory that allows a user to electrically program and erase information. An EEPROM memory device typically includes EEPROM memory cells arranged in an array and peripheral circuits around the array for accessing the memory cells for program or erase operations. The capacity of an EEPROM memory device, i.e., the amount of information the memory device is capable of storing, is typically associated with the number of memory cells in the memory device. But a larger number of memory cells in a memory device also means a larger size for the memory device and possibly slower circuit performance. Therefore, smaller EEPROM cells are generally desired, and much effort has been made to create innovative EEPROM cell designs in order to produce EEPROM memory devices with large capacity, small size, and good circuit performance.